Cascaded monolithic crystal filter with high out-of-band rejection

ABSTRACT

In one embodiment, a cascaded monolithic crystal filter is provided. A first filter includes two resonators having a pair of electrodes with the monolithic crystal between. At least one electrode has a periphery which includes a feature capable of shifting a frequency associated with an anharmonic mode in the filter. The filter has a second resonator acoustically coupled to the first resonator. A second filter is cascaded with the first filter. The second filter includes a pair of acoustically coupled resonators.

BACKGROUND

MCF (monolithic crystal filter) based on quartz technology operating inthe fundamental shear mode have been used over the past half century forradio communication applications. The center frequency of these filterare limited to about 10-250 MHz, due to the fabrication difficulty. Inthe last decade, the rapid progress in the wireless communication hascreated a strong demand for high performance Gigahertz filters withsmall dimension and low power consumption.

Currently, the commercially available monolithic crystal filters areusually limited to center frequency of about 250 MHz due to thefabrication difficulties. These relatively low frequency filters showeda rather high insertion loss of 6-7 dB when good out-of-band rejectionof about 60 dB is required. At high frequency, the difference in thestrength of the first two undesirable anharmonic modes observed in aresonator are usually less than 10 dB from the desirable fundamentalshear mode. The suppression of these anharmonic modes has become acritical issue for a high performance Giga Hertz MCF.

To extend the existing MCF technology into the GHz range is not trivial.For a low frequency MCF, a good out-of-band rejection may be achieved bydesigning a quartz filter supporting only fundamental shear mode.However, at high frequency, greater than 1 GHz, a single mode quartzfilter will have the dimension of smaller than a few microns by a fewmicrons, and a gap of 1 micron or less. The tolerance for thefabrication error for such a filter may be costly and difficult toattain with conventional processing. Therefore, a highly effectivetechnique for suppression of anharmonic modes is necessary.

Thus, what is needed is a high performance Giga hertz MCF capable ofeffectively suppressing anharmonic modes.

SUMMARY

In one embodiment, a cascaded monolithic crystal filter is provided. Afirst filter includes two resonators having a pair of electrodes withthe monolithic crystal between. At least one electrode has a peripherywhich includes a feature capable of shifting a frequency associated withan anharmonic mode in the filter. The filter has a second resonatoracoustically coupled to the first resonator. A second filter is cascadedwith the first filter. The second filter includes a pair of acousticallycoupled resonators.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be betterunderstood with regard to the following description, appended claims,and accompanying drawings where:

FIG. 1 is a side view of a simplified illustration of one example of atypical 2-pole monolithic crystal filter.

FIG. 2 shows a plot illustrating the spectral response of a typical2-GHz 2-pole MCF with dimensions of about 15 microns by 19 microns.

FIG. 3A is a top view illustrating the acoustic energy of thefundamental mode (1,1,1) for a resonator.

FIG. 3B is a top view illustrating the acoustic energy of the anharmonicmode (1,1,3) for a resonator.

FIG. 3C is a top view illustrating the acoustic energy of the anharmonicmode (1,3,1) for a resonator.

FIG. 4 is a top view showing a simplified illustration of monolithiccrystal filters with and without electrode tabs.

FIG. 5 is a plot of the spectral responses of monolithic crystal filterof FIG. 4 with tabs and without tabs.

FIG. 6 is a top view showing a simplified illustration of monolithiccrystal filters with and without electrode tabs.

FIG. 7 is a plot of the spectral responses monolithic crystal filter ofFIG. 6 with tabs and without tabs.

FIG. 8 is a top view of showing a simplified illustration of twocascaded monolithic crystal filters modified with the electrode tabsshown in FIGS. 4 and 6.

FIG. 9 is a plot of the spectral responses of the cascaded monolithiccrystal filter of FIG. 8.

FIG. 10 is a top view of showing a simplified illustration of twocascaded monolithic crystal filters.

FIG. 11 is a plot of the spectral responses of the cascaded monolithiccrystal filter of FIG. 10.

DESCRIPTION

FIG. 1 is a side view of a simplified illustration of one example of atypical 2-pole MCF or monolithic crystal filter 100. The electrodes 112and 122 are separated from electrodes 113 and 123, respectively, by apiezoelectric material 130, typically quartz. An input voltage signal at110 is coupled to the output 120 by acoustical coupling through thepiezoelectric material 130.

FIG. 2 shows a plot illustrating the spectral response 200 of a typical2-GHz 2-pole MCF with electrodes separated by 1.5 microns and havingdimensions of approximately 15 microns by 19 microns. The out-of-bandrejection for this filter is very poor due to the existence of severalstrong undesirable anharmonic modes, especially (1, 1, 3) and (1, 3, 1),in addition to a desirable fundamental shear mode (1, 1, 1). A simplecascade of MCF of the same kind will not significantly improve theout-of-band rejection. An effective way of suppressing anharmonic modes,while keeping the insertion loss of the filter low, is to cascade two ormore filters with the same fundamental frequency but significantlydifferent in the anharmonic modes. To achieve this, the differences inthe characteristics of acoustic energy trapping of the fundamental andanharmonic modes is exploited as discussed below.

FIGS. 3A-C are top views illustrating the acoustic energy trappings ofthe fundamental mode (1,1,1), shown in FIG. 3A, and anharmonic modes(1,1,3) and (1,3,1), shown in FIGS. 3B and 3C respectively, for aresonator 300. FIGS. 3A-C illustrate how features, such as tabs 360 at ₁and/or 360 at ₂ (FIG. 3A), and/or cut-out portions 360 ar ₁ and/or 360ar ₂ (FIG. 3A), formed along the periphery 360 of the electrode 312affect the fundamental mode (1,1,1) shown in FIG. 3A, differently fromthe anharmonic modes (1,1,3) and (1,3,1) shown in FIGS. 3B and 3C.

The (1, 1, 1) mode, shown in FIG. 3A, has acoustic energy confined inthe center of the electrode 312 a. As such, the fundamental mode (1,1,1)is insensitive to the perturbation of the periphery 360 a of theelectrode 312 a. As a result, adding a small electrode tab 360 at ₁and/or 360 at ₂, or removing a small portions 360 ar ₁ and/or 360 ar ₂of electrode 312 a, by laser trimming or FIB for example, will haveessentially no effect on the fundamental mode (1, 1, 1) shown in FIG.3A.

The anharmonic modes (1,1,3) and (1,3,1), however, have acoustic energyspreading toward the peripheries 360 b and 360 c of the electrodes 312 band 312 c, respectively. As such, the anharmonic modes (1,1,3) and(1,3,1) are very sensitive to perturbation of the electrode periphery360 b and 360 c, respectively. Adding tabs 360 bt ₂ and 360 ct ₁, orremoving a small portions 360 br ₂ and 360 cr ₁ of the electrode 360 band 360 c has a significant effect on the resonance frequency ofrespective anharmonic modes. For modes (1,1,3), (1,1,5), etc., addingthe tab 360 bt ₂ will shift the (1,1,3) (1,1,5) mode toward a lowerfrequency than an unperturbed electrode. For modes (1,1,3), (1,1,5),etc., removing a portion 360 br ₂ will shift the (1,1,3) (1,1,5) towarda higher frequency than unperturbed electrode. Such a modification toelectrode, however, will have negligible effect on the (1, 1, 1) and (1,3, 1) modes. Similarly, adding tab 360 ct ₁, or removing portion 360 cr₁ has the significant effect on modes (1,3,1), (1,5,1), etc., but not onmodes (1, 1, 1) and (1, 1, 3).

FIG. 4 is a top view showing a simplified illustration of electrodes 411and 412 of monolithic crystal filters 405 and 415. Shown in FIG. 5 is aplot of the spectral response of filter 405 (pair of acousticallycoupled resonators) with electrode tabs 460 t along with a plot offilter 415 (pair of acoustically coupled resonators) without tabs, shownin FIG. 4. In the specific example embodiment of FIG. 4 the electrodes412 are separated by 2 microns and are each 16 microns by 16 micronswith tabs 460 t that extend 1.6 microns from the periphery of theelectrode 412 and are each 4 microns wide. The electrodes 411 aresimilarly sized and spaced, but without tabs.

FIG. 6 is a top view showing a simplified illustration of electrodes 611and 612 of monolithic crystal filters 605 and 615. FIG. 7 is a plot 700of the spectral response of filter 605 (pair of acoustically coupledresonators) with electrode tabs 660 t along with a plot of filter 615(pair of acoustically coupled resonators) without tabs, shown in FIG. 6.In the embodiment of FIG. 6 the electrodes 612 are separated by 2microns and are each 16 microns by 16 microns with tabs 660 t thatextend 2 microns from the periphery of the electrode 612 and are each3.2 microns wide. The electrodes 611 are similarly sized and spaced, butwithout tabs

Referring to FIGS. 5 and 7, in both plots 500 and 700, the desirable (1,1, 1) mode remains essentially unperturbed. However, the anharmonicresonance frequencies of (1, 1, 3) and (1, 3, 1) modes, respectively,have been shifted downward significantly by the acoustically coupledresonators 405 and 605 with the tabs 460 t and 660 t, respectively.Thus, the shifted anharmonic frequencies can be filtered by cascading.

FIG. 8 is a top view of a simplified illustration of a 2 pole cascadedmonolithic crystal filters embodiment 800. This example embodimentimproved out-of-band rejection to greater than 45 db as shown in thespectral response plot 900 of FIG. 9. In the embodiment of FIG. 8, theelectrodes 811 are separated by 1.5 microns and are each 15 microns by19.2 microns, with tabs 860 t that extend 1.5 microns from opposite 19.2micron sides of the electrodes 811 and are each 4.8 microns wide.Similarly, the electrodes 812 are separated by 1.5 microns and are each15 microns by 19.2 microns, but with tabs 861 t that extend 1.2 micronsfrom adjacent 15 micron sides of the electrodes 812 and are each 6microns wide. In this embodiment, the tabs 860 t are shown on oppositesides of the electrodes 811 and the tabs 861 t are shown on adjacentsides of the electrodes 812.

FIG. 10 shows another example embodiment of a cascaded monolithiccrystal filter 1000. In this example, the out-of-band rejection wasfurther improved to greater than 55 dB with less than 4 db insertionloss. In the embodiment of FIG. 10, the electrodes 1011 are separated by1.5 microns and are each 15 microns by 15 microns, with tabs 1060 t thatextend 1.5 microns from opposite sides of the pair of electrodes 1011,and are each 9.6 microns wide. Similarly, the electrodes 1012 areseparated by 1.5 microns and are each 15 microns by 19.2 microns, butwith tabs 1061 t that extend 1.2 microns from adjacent 15 micron sidesof the electrodes 1012 and are each 6 microns wide. FIG. 11 shows thespectral plot 1100 for the example embodiment of FIG. 10.

In the cascaded monolithic crystal filter embodiments 800 (FIG. 8), forexample, the cross sectional area of the tabs 860 t and 861 t have thesame cross sectional area to ensure that the fundamental mode remainsunchanged, while all of the anharmonic modes are shifted.

In various embodiments discussed above, monolithic crystal filters maybe easily fabricated to provide a significant difference in anharmonicmodes, while the center frequency remains essentially unchanged.Cascading two or more of these filters can provide a band-pass MCF withlow insertion loss (a few dB or less) and extreme high out-of-bandrejection (70-80 db or more).

Further, various embodiments may provide extremely low insertion lossfor filters up to a few GHz regardless of extremely narrow (much lessthan 1% of the center frequency) or very wide (greater than 10%)bandwidth due to an extremely high Q. Moreover, a steep and highout-of-band attenuation is possible. In addition, some embodiments canprovide minimum ripple in transmission band (much less than 1 dB).

In certain applications, embodiments can be used to provide a passivefilter, with no other power consumption, other than insertion loss.Further, embodiments may be easily miniaturized if desired. Thus,embodiments may also have a great potential for wireless communicationapplication into a small, low cost component.

Although above embodiments are shown with tabs, other embodiments mayhave cut-outs of the peripheral edge of the electrodes instead of, or inaddition to tabs. Furthermore, although generally rectangular tabs andcut-outs are shown for illustration purposes, other shapes,configurations, or features are possible, such as for example, arcuate,circular, tapered, triangular, trapezoidal, etc., or other features ator near the periphery of the electrode. A “cut-out” is as used hereinmay be formed during deposition without having to remove material bycutting, etching, or other removal technique. Also, a tab may be formedfrom an electrode by cutting, trimming, etching, or other removaltechnique, or be added to an electrode after electrode formation.

In addition, although one tab or cut-out is shown on each electrode inFIGS. 4, 6, 8, and 10, it is possible that each electrode have more thanone. For example, multiple tabs or cut-outs may be located on a sameedge, or on opposing edges, of an electrode.

In alternate embodiments, a conventional monolithic crystal filter 100,shown in FIG. 1, may include tabs and/or cut-outs. This can reduce theanharmonic signal by 5 to 10 dB in some such embodiments. Although notrequired in all embodiments, typically, resonator electrode pairs, suchas 112 and 113, will have the same feature at their peripheries. In onepossible alternate embodiment, electrodes 112 and 113 may be configuredas represented in FIG. 3B, and the electrodes 122 and 123 may beconfigured as represented in FIG. 3C. Or, in another alternateembodiment, electrodes 112 and 113 may be configured as shown in FIG.3A, and electrodes 122 and 123 without any feature at the periphery.

The example embodiments herein are not intended to be limiting, variousconfigurations and combinations of features are possible.

Having described this invention in connection with a number ofembodiments, modification will now certainly suggest itself to thoseskilled in the art. As such, the invention is not limited to thedisclosed embodiments, except as required by the appended claims.

1. A cascaded monolithic crystal filter comprising a pair of cascaded filters each comprising acoustically coupled resonators, wherein the pair of cascaded filters each comprise at least one resonator with at least one electrode having a periphery comprising a feature capable of shifting a frequency of an anharmonic mode without substantially shifting a frequency of a fundamental mode of the filter, and wherein the feature of each of the pair of filters is configured such that one of the pair of filters is capable of shifting a frequency of a different anharmonic mode than an other of the pair of cascaded filters.
 2. The filter of claim 1, wherein the feature comprises at least one of: (a) a tab, or (b) a cut-out.
 3. The filter of claim 1, wherein both electrodes of at least one of the resonators comprise the feature at a periphery.
 4. A cascaded monolithic crystal filter comprising: a) a monolithic crystal; b) a first filter comprising: i) a first resonator comprising: (1) a pair of electrodes with the monolithic crystal therebetween; and (2) at least one of the pair of electrodes having a periphery comprising a feature capable of shifting a frequency associated with an anharmonic mode in the first filter; and ii) a second resonator acoustically coupled to the first resonator; and c) a second filter cascaded with the first filter, the second filter comprising: i) a third resonator comprising: (1) a pair of electrodes with the monolithic crystal therebetween; and (2) at least one of the pair of electrodes of the third resonator having a periphery comprising a feature capable of shifting a frequency associated with an anharmonic mode so as to cause separation between the anharmonic modes of the first filter and the second filter; and ii) a fourth resonator acoustically coupled to the third resonator.
 5. The filter of claim 4, wherein the second resonator comprises an electrode having a periphery comprising a feature capable of shifting a frequency associated with an anharmonic mode in the first filter.
 6. The filter of claim 5, wherein the fourth resonator comprises an electrode having a periphery comprising a feature capable of shifting a frequency associated with an anharmonic mode in the second filter.
 7. The filter of claim 4, wherein the fourth resonator comprises an electrode having a periphery comprising a feature capable of shifting a frequency associated with an anharmonic mode in the second filter.
 8. The filter of claim 4, wherein the feature of the first resonator and the feature of the third resonator comprise at least one of: (a) a tab, or (b) a cut-out.
 9. The filter of claim 4, wherein both electrodes of the pair of electrodes of the first resonator comprise the feature at the periphery.
 10. A cascaded monolithic crystal filter comprising: a) a monolithic crystal; b) a first filter comprising: i) a first resonator comprising: (1) a pair of electrodes with the monolithic crystal therebetween; and (2) at least one of the pair of electrodes having a periphery comprising a feature capable of shifting a frequency associated with an anharmonic mode in the first filter; and ii) a second resonator acoustically coupled to the first resonator; c) a second filter cascaded with the first filter, the second filter comprising: i) a third resonator comprising an electrode having a periphery comprising a feature capable of shifting a frequency associated with an anharmonic mode in the second filter; and ii) a fourth resonator acoustically coupled to the third resonator, wherein the fourth resonator comprises an electrode having a periphery comprising a feature capable of shifting a frequency associated with an anharmonic mode in the second filter; and d) wherein the feature of the first filter and at least one of: (1) the feature of the third resonator; or (2) the feature of the fourth resonator are configured such that the first filter is capable of shifting a frequency associated with a different anharmonic mode than the second filter.
 11. The filter of claim 10, wherein the feature of the first resonator, the feature of the third resonator, and the feature of the fourth resonator comprises at least one of: (a) a tab, or (b) a cut-out.
 12. A cascaded monolithic crystal filter comprising a pair of cascaded filters each comprising acoustically coupled resonators, each of the pair of cascaded filters each comprising at least one electrode having a periphery comprising a feature capable of shifting a frequency of an anharmonic mode without substantially shifting a frequency of a fundamental mode of the filter, and wherein the feature of each of the pair of cascaded filters causes separation between the anharmonic modes of each of the pair of filters so as to provide a band-pass monolithic crystal filter.
 13. The filter of claim 12, wherein the feature comprises at least one of: (a) a tab, or (b) a cut-out.
 14. A method for a monolithic crystal filter comprising: a) shifting an anharmonic mode frequency of a first filter by providing a feature at a periphery of at least one electrode of a first filter resonator; b) cascading the first filter with a second filter so as to filter an anharmonic mode frequency; c) shifting an anharmonic mode frequency in the second filter; and d) wherein shifting the anharmonic mode frequency of the second filter comprises shifting a different anharmonic mode frequency than the first filter.
 15. The method of claim 14, wherein providing a feature comprises providing at least one of: (a) a tab; or (b) a cut-out.
 16. The method of claim 14, wherein shifting an anharmonic mode frequency of the first filter comprises providing the feature at a periphery of both electrodes of the first filter resonator.
 17. A method for a monolithic crystal filter comprising: a) shifting an anharmonic mode frequency of a first filter by providing a feature at a periphery of at least one electrode of a first filter resonator without substantially shifting a frequency of a fundamental mode of the first filter; b) shifting an anharmonic mode frequency in the second filter by providing a feature at a periphery of at least one electrode of a second filter resonator so as to cause frequency separation between the anharmonic modes of the first filter and the second filter without substantially shifting a frequency of a fundamental mode of the second filter; and c) cascading the first filter with a second filter so as to provide a band-pass monolithic crystal filter.
 18. The method of claim 17, wherein providing a feature in the first filter and providing a feature in the second filter comprises providing at least one of: (a) a tab; or (b) a cut-out. 